Arctic Defrost?

This September, the NSIDC (the US National Snow and Ice Data Center) revealed that the extent of the September sea ice in the Arctic this year was at the lowest point since measurements started in 1979 (a link to the press release can be found here). 

An image of the extent of sea ice in September 2012. The pink line is the average extent around this time over the period of 1979-2000 (NSIDC)

The record is in line with observations over the last decades, showing that there has been a steady decline in summer sea ice extent. Scientists believe that this trend is largely due to higher temperatures as a result of global warming.

A graph comparing sea ice extent between 2012, 2007 (the previous record-low) and the 1979-2000 average (from 30 October 2012 - NSIDC website)

Another observation is the decrease in thickness of the ice. The ocean water absorbs more heat, and slowly releases it when the sun sets - making it more difficult for the ice to regrow. Most of the ice is now only 1 or 2 years old, and is more susceptible to melting. Moreover, the disappearance of the ice means that less sunlight can be reflected back, resulting in warmer conditions. The positive albedo effect is working here as well.

It was previously thought from models from the IPCC report from 2007 that the Arctic might become ice free in summers by 2100. However, these recent data seem to indicate that the decline in ice is progressing faster than previously thought. Scientists now believe that summer ice might disappear completely from the Arctic region in a few decades.

I want to conclude this short update on the state of the Arctic sea ice with an informative clip from Climate Watch Magazine, where the sea ice has been animated based on the observations from 1979 onwards. It also shows the age of the ice in different colours - note how the 1 year old ice starts to dominate towards the end. It can be found here (I'd insert it - but it's a bit too big!).

In a following post, I'll give more attention to albedo feedback and ice, and I'll also discuss the effect of soot on ice reflectivity, something that's become more important in recent years.

To Plant or not to Plant? Reforestation and Albedo

For thousands of years, humans have cleared land for agriculture, cutting down trees to make space for croplands. Deforestation is an ongoing process; in the tropics especially, trees are cut down or even burned at rapid pace.

Forests play an important role in climate, especially in the hydrological and carbon cycles. Trees control humidity (and thus temperature) through evapotranspiration, and it has been found that deforested areas in the Amazon area have become both drier and warmer. Moreover, a growing forest fixes carbon from the atmosphere - cutting down trees releases it. Reforestation has been suggested as an option to sequester carbon, and thereby reduce the carbon dioxide in the atmosphere and counter global warming.

So how does albedo fit into this? As explained in earlier posts, different surfaces have a different albedo. A good example of this can be found in the figure below, derived from an article by Bonan in 2008. As can be seen, the albedo of forests is generally lower than that of other biomes. This is due to the fact that forests are quite dark and shady, and they do not reflect much of the solar radiation.

Surface albedo of different forest biomes (Bonan 2008)

The question scientists have been asking is whether this 'warming' effect offsets the cooling benefits of a forest. It seems as though this varies per type of forest. Basically, we can identify three types; tropical, temperate and boreal. It was found that evapotranspiration is very important in tropical forests. Warmer air can contain more water vapour and so more heat can be removed in that way (this is called latent heat transport). As said above, cutting down trees in Amazonia led to a warmer and drier climate. Additionally, rain forests grow rapidly and are able to store much carbon.

In boreal forests, snowfall also plays a role. Research has shown quite clearly that snow on bare ground has a higher albedo than snow on trees (which makes sense; imagine a forest covered in snow compared to a field. The trees will still show dark patches, whereas the field is much more uniformly covered). In addition to that, the evaporative effect of boreal vegetation is much smaller, because the air is generally colder. Boreal forests also grow slower, so carbon storage benefits from reforestation would require more time to take effect.

For temperate forests, the effects of albedo and evaporative forcing are unclear - and it is not certain whether replanting those forests might actually help in negating global warming.

In a model run by Giddard et al. in 2005, it was found that changes in surface (so either forestation or deforestation) particularly impact the higher latitudes in the Northern Hemisphere (also see the figure below). They found that the albedo "side-effect" of reforestation would lead to a net-warming on a century timescale, offsetting any benefits of carbon sequestration.

Modelled changes in albedo (Gibbard et al. 2005)

As such, it has been concluded that reforestation of tropical areas has some use in counteracting global warming, temperate forests little to no effect, and replanting boreal forests actually exacerbates it. Nevertheless, though it is tempting to draw such a one-on-one relation between forests, albedo and climate, trees and forests have other effects on climate as well, on various spatial and temporal scales, not to mention the importance of forests for biodiversity and human culture. There is much research needed in modelling all interactions between forests and the climate system before it can conclusively be said whether (and where) reforestation can help mitigate global warming.

Sources: 

• Bonan, G.B. (2008), Forests and climate change: Forcings, feedbacks and the climate, Science, 320, 1444, doi:10.1126/science.1155121.
• Gibbard, S., K. Caldeira, G. Bala, T. J. Phillips, and M. Wickett (2005), Climate effects of global land cover change, Geophys. Res. Lett., 32, L23705, doi:10.1029/2005GL024550.

Uneven Distribution: Local Albedo

At this point, it is good to note that the light from the sun doesn't hit the Earth's surface evenly.

Source: Nature Education 

Over some parts of the Earth, the same amount of sunlight covers a larger area due to the spherical shape of the planet (also see picture above). This is one of the principal drivers of climate differences between different latitudes, and why it's warmer at the equator than at the North Pole.

This relates to albedo as well: Even though snow may have an albedo of up to 0.8 for pristine snow, the amount of light reflected back is still not very much because it was never a lot to begin with. Conversely, a tropical forest is dark, shady and absorbs a lot of sunlight. Its albedo will never be more than 0.2. Nevertheless, because the amount of incoming sunlight at the equator is so high, the absolute amount of reflected light might be more than at the poles. Local differences such as these can impact the planet's total albedo.

On average, the albedo of the planet Earth is about 0.39, even though the oceans have a far lower albedo and cover most of the surface. Clouds are an important contributor: as white sheets floating in the sky, they play an important role in returning sunlight before it has even hit the ground (a situation that can only too often be observed in real life in London).

More will follow soon on clouds, forests, deserts and other local albedo factors.

Albedo Albedo Albedo: What's it all about?

This blog is about the albedo effect; according to the Oxford Dictionary, it is "the proportion of the incident light or radiation that is reflected by a surface, typically that of a planet or moon". That might sound more complex than it actually is. What it comes down to, is that some surfaces are more reflective than others.

Think of snow. Anyone who's walked around a pristine field of snow in the sun will remember barely being able to look at it. The snow reflects the sun. Other surfaces, for example asphalt, actually absorb the sunlight. They are not hard to look at on a bright day, but they can become very hot. There is a fairly simple property that governs the albedo of a certain surface, namely colour. Basically, lighter colours reflect more light than darker ones, which is why white snow is so much more difficult to look at than black asphalt.

For the planet, this means that there are some parts that act like a mirror, reflecting the incoming sunlight back into space. Ice sheets have a high albedo, meaning that they sent back much light, as do clouds. Dark ocean water, forests and most human built structures have a low albedo. They absorb more light than they reflect back.

Source: http://www.climatepedia.org/Albedo

The albedo is the percentage of light reflected. Of course, it makes sense that when we change the surfaces of the Earth, because of global warming or deforestation, the Earth's albedo will change as well. Less ice will mean less sunlight being reflected back into space, and more light being trapped on Earth - which means more heat as well and more global warming leading to more ice melting, leading to lesser albedo of the Earth's surface... In other words, through the albedo effect, global warming could potentially be exacerbated.